The feasibility of immunization against primate lentivirus infection and disease has been demonstrated. However, a number of key questions remain unsolved. These include (1) What are the relevant properties of HIV-1 isolates on which to base vaccine designs? (2) How to elicit broadly neutralizing antibody responses against primary HIV-1 isolates and (3) How does the "prime and boost" immunization work and how to refine this approach to augment both T and B-cell mediated immunity against HIV? Answers to these questions will provide better insights into the design of effective vaccines against HIV and AIDS. The overall objective of this proposal is to critically examine these questions. We hypothesize that (1) there are HIV-1 genomes that encode more optimal sequences for vaccine design than viruses that are the base for current vaccines; (2) neutralizing antibodies are essential components of protective immunity; (3) different immunization modalities activate different immune responses and that combinations of these modalities ("prime and boost") offers the best likelihood of inducing the balanced immune response required for protection. These hypotheses are supported by the work accomplished by a number of well-established investigators participating in this Program. This new application builds upon the existing interactions of these investigators in the greater Seattle area and benefits from the combined resources and expertise from academic and private institutions. The Specific Aims of this proposal are: (1) To identify non-subtype B viruses that have biological and immunological properties suitable for vaccine development in sub-Saharan Africa (Project 1: Overbaugh); (2) To generate broadly neutralizing immune responses by "quasispecies" envelope vaccines (Project 2: Haigwood); (3) To examine envelope modifications as an approach to elicit broadly neutralizing antibodies (Project 3: Stamatatos); and (4) To analyze and refine the 'prime and boost" immunization strategy to augment both T and B cell mediated immunity against SHIV (Project 4: Hu/Kaja/Morrow). These efforts will be supported by four Cores: Core A (Anderson) will provide non-human primate resources and related expertise; Core B (Hu/Firpo) will provide assays and laboratory supports for all macaque studies; Core C (Grabstein) will provide immunogens and formulation technologies; and Core D (Hu) will provide the administrative and biostatistics support for the entire Program. Results and material generated from this Program are likely to contribute directly to clinical development of AIDS vaccines and enhance our understanding of the protective mechanisms and effective immunization strategies against primate lentiviruses.